1. Field of the Invention
This invention relates to receiver systems. In particular the invention relates to receiver systems for mobile telephone systems.
2. Description of the prior art
With the impending adoption of the GSM (Groupe Speciale Mobile) system for a Pan-European Mobile Telephone System [see for example Electronics and Communication Engineering Journal January/February 1989 vol. 1 no. 1 pp. 7 to 13, "Pan-European cellular radio", D. M. Balston], a number of difficult technical problems have to be overcome for satisfactory implementation of the system.
In mobile telephone systems such as GSM, or PCN (Personal Communications Network), data to be transmitted is modulated on to a UHF band (300 MHz-3 GHz) carrier utilising the digital modulation techniques, e.g. Gaussian minimum shift keying. In GSM, frequency channels are provided at a spacing of 200 KHz and data is transmitted on each channel at a total rate of 270.833 Kbits/s.
Referring to FIG. 1, this shows the basic format of one frequency channel for mobile telephone systems transmissions as comprising a series of frames (Time Domain Multiple Access Frames), each frame comprising eight multiplexed time slots from different mobile stations. Each frame is 4.615 ms in length and each time slot 0.577 ms in length. The structure of a time slot as shown comprises two sections of data separated by control bits, a training sequence, tail bits etc. Data is transmitted from each mobile station within a time slot in compressed format at a rate of 13 kbits.sup.-1.
Given the very tight technical constraints of mobile telephone systems, interference and data loss and corruption can easily occur.
A major problem is that of multipath, since the transmissions reflect from buildings, hills, high-sided vehicles etc. Many different reflections can reach the same position of a mobile or base station, however, and even when there is a direct path it is not unknown for strong reflections to be received as well. The radio paths taken by the reflections are longer than the direct path and at the bit rates chosen for mobile telephone systems the difference in path length can be equivalent to several bit periods. FIG. 2 demonstrates this effect, and it can be seen that the combined signal received at the mobile's antenna can be severely corrupted.
To date existing radio systems have substantially avoided this multipath effect by choosing bit rates which are long compared with the expected multipath delays. In mobile telephone systems, equalisation is employed to recover the wanted signal despite severe multipath corruption.
Equalisation works by making an estimate of the impulse response of the transmission medium and then constructing an inverse filter through which the received signal is passed. There are several methods for estimating the transfer function of the transmission path and several variations of algorithm associated with each, but whatever the method they all rely on receiving a known sequence of data. This is the training sequence which is transmitted in the middle of each time slot. The receiver detects this sequence, and, knowing what bit pattern it represents, is able to estimate the transfer function most likely to have produced the signal received. The calculation of the coefficients of the filter required to compensate for the response is then relatively straightforward.
The multipath effects can change very rapidly in practice. The wavelength at 900 MHz is only 30 cm and thus a change in the differential path length of only 15 cm between two signals received at an antenna can change their interference from constructive to destructive. The mobile telephone system specifications are designed to accommodate vehicles moving at up to 250 km/h and thus the mobile could have moved up to 32 cm in the 4.6 ms between successive traffic channel time slots. Add to this the problems of reflections from other moving vehicles and it is clear that each time slot has to be treated independently. The equalisation technique commonly employed is based upon the Viterbi algorithm.
One particular problem which arises from multipath is that of fading which is caused by destructive interference between two signals taking different paths to the receiver.
As mentioned above, a mobile may have moved a distance in the 4.6 ms between successive channel time slots of up to 32 cm, which is about the same as the wavelength of 900 MHz radiation. Thus interference may have changed from constructive to destructive within the space of adjacent channel time slots.